Summary:
The Nicotiana tabacum UXS3 gene encodes a protein (AtUXS3) with UDP-glucuronate decarboxylase activity [Harper02], involved in the conversion of UDP-glucuronate into UDP-xylose. Unlike AtUXS1 and AtUXS2, this enzyme does not possess a hydrophobic N-terminal domain thought to anchor the protein in endomembranes; the C-terminus moiety of the protein supposedly faces the lumen of endomembrane compartments [Harper02]. AtUXS3 has a very high substrate specificity for UDP-GlcA. UDP-glucose, UDP-galactose, UDP-mannose, GDP-glucose, GDP-mannose and UDP-galacturonate were not used as substrates [Harper02]. This enzyme does not require exogenously added NAD+, although the cofactor is required for activity. Moreover, the addition of NADH, NADPH or NADP+ failed to inhibit the activity of AtUXS3. It is therefore thought that NAD+ is tightly bound to the protein [Harper02]. AtUXS3 has a broad pH range (pH 4.5 to 9.5) but is completely inactive at pH values below 3.2 (interestingly, at this pH, preincubation of the enzyme with NAD+ retains 60% of enzyme activity [Harper02]). In addition, the enzyme also displays a broad temperature range: 22?C to 42?C. No activity was detected beyond 55?C.